1. Field of the Invention
The present invention relates to a force sensor which detects at least one of a force and a moment applied to an operating section on the basis of the value of an output voltage of a magnetic-electric conversion element, such as a Hall element, when an external force acts on the operating section, causing the magnetic-electric conversion element to be relatively displaced with respect to a magnet. Further, the present invention relates to a robot apparatus in which a portion connecting the distal end of a multi-joint robot arm and an end effector is provided with a sensor main body of a force sensor and which allows the posture and the driving force of a robot arm to be controlled by the force detected by the force sensor, thus achieving smooth assembly.
2. Description of the Related Art
When assembling components or members by an automatic assembling apparatus formed of a robot apparatus, the assembling operation cannot be smoothly achieved unless each component or member is accurately positioned by a robot hand of the robot apparatus. For example, in the fitting assembly operation for gears, pins or the like, even slightest misalignment of an axial center has been preventing vertical insertion from being accomplished, resulting in an assembly failure.
To solve the problem described above, there is a robot apparatus in which the section connecting the distal end of a robot arm of the robot apparatus and an end effector is provided with a force sensor for detecting a force in the direction of each of an X-axis, a Y-axis and a Z-axis applied between the robot arm and the end effector and a moment about each of the axes. The posture and the driving force of the robot arm are controlled on the basis of the forces and moments detected by the force sensor, thus permitting smooth assembly.
A predominant conventional force sensor is a type in which strain gauges are provided on a flexible beam connecting a frame and an operating section, and a force and a moment are detected on the basis of change in the resistance of the strain gauges. However, in a force sensor using strain gauges, a plurality of strain gauges is disposed on the same beam. Hence, when a flexure portion is subjected to a force, there is other axis interference, in which a strain occurs not only in the direction of an axis in which a force is applied but a strain occurs also in the direction of axis in which the force is not applied. It is difficult, in principle, to achieve a beam construction that is not affected by forces in the directions of any other axes in order to minimize the influences of the other axis interference. For this reason, it has been required to carry out post-processing on the amount of interference of a component of another axis on the basis of a detected signal thereby to, for example, subtract the influence by devising signal processing or the like in a subsequent stage. This has been disadvantageous in achieving a reduced size and lower cost.
Meanwhile, there has been proposed a force sensor adapted to magnetically detect the displacement of an operating section (refer to Japanese Patent Application Laid-Open No. 2004-325328). In the force sensor, four Hall elements are disposed, facing against a pole surface of a permanent magnet embedded in an elastic member. When an acting force is applied, the permanent magnet is displaced. A change in a magnetic flux attributable to the displacement is detected by magnetic-electric conversion elements, such as the Hall elements. This permits the detection of the force components in the directions of three axes, namely, in the direction of the X-axis, the direction of the Y-axis, and the direction of the Z-axis. An electromagnet may be used in place of the permanent magnet. For the convenience of explanation, Hall elements may be used to represent the magnetic-electric conversion elements in the following description. This, however, should not be deemed that the magnetic-electric conversion elements are limited only to Hall elements.
Each of a permanent magnet and an electromagnet (hereinafter referred to as “the magnet” in some cases) has a predetermined temperature coefficient in relation to the intensity of a magnetic field to be generated. Hence, the intensity of a magnetic field changes according to the influences of temperature or heat in a surrounding environment of the sensor, including a change in an environmental temperature, the heat from a motor, which is driving force source for a robot, or heat from a signal processor in a circuit board. Further, regarding a magnetic-electric conversion element, such as a Hall element or an MR element, the output voltage thereof (the Hall voltage in the case of a Hall element), which is proportional to a magnetic flux, changes as the environment temperature changes. In other words, as the environmental temperature changes, the detection sensitivity of a magnetic-electric conversion element changes.
Meanwhile, there has been proposed a method for making a temperature-based correction for a Hall element by a temperature compensation circuit having a temperature sensor (refer to Japanese Patent Application Laid-Open No. 2005-321592). According to the method, if the output voltage of a Hall element falls due to a temperature rise, then the current supplied to the Hall element is increased in proportion to a temperature rise due to a negative temperature characteristic of the temperature sensor. The increased current output to the Hall element regains the output voltage that has decreased due to the temperature rise.
To correct the temperature of a magnetic-electric conversion element by using a temperature compensation circuit, it is necessary to compensate for a change in an output voltage, which is a detection magnetic flux, by providing a temperature sensor also in the vicinity of a magnet, because not only the characteristics of the magnetic-electric conversion element but also the magnetic flux of the magnet changes when an environmental temperature changes.
It is difficult, however, to accurately measure the average temperature of the magnet by using the temperature sensor because of the difference in heat capacity between the magnet and the temperature sensor or the difference in the positional relationship between the temperature sensor and the magnet when a local temperature rise takes place. Further, it would be required to provide the magnet and the magnetic-electric conversion element with temperature sensors exclusively used for correcting temperature characteristics, thus complicating the circuit.